Network Working Group Stanislav Shalunov
Internet Draft Benjamin Teitelbaum
Expiration Date: June 2005 Anatoly Karp
Jeff W. Boote
Matthew J. Zekauskas
Internet2
December 2004
A One-way Active Measurement Protocol (OWAMP)<draft-ietf-ippm-owdp-14.txt>
Status of this Memo
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patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Copyright Notice
Copyright (C) The Internet Society 2004. All Rights Reserved.
Abstract
With growing availability of good time sources to network nodes, it
becomes increasingly possible to measure one-way IP performance
metrics with high precision. To do so in an interoperable manner, a
common protocol for such measurements is required. The One-Way
Active Measurement Protocol (OWAMP) can measure one-way delay, as
well as other unidirectional characteristics, such as one-way loss.
Shalunov et al. [Page 1]

INTERNET-DRAFT One-way Active Measurement Protocol December 20041. Introduction
The IETF IP Performance Metrics (IPPM) working group has proposed
draft standard metrics for one-way packet delay [RFC2679] and loss
[RFC2680] across Internet paths. Although there are now several
measurement platforms that implement collection of these metrics
[SURVEYOR] [RIPE] [BRIX], there is not currently a standard that
would permit initiation of test streams or exchange of packets to
collect singleton metrics in an interoperable manner.
With the increasingly wide availability of affordable global
positioning systems (GPS) and CDMA-based time sources, hosts
increasingly have available to them very accurate time
sources--either directly or through their proximity to Network Time
Protocol (NTP) primary (stratum 1) time servers. By standardizing a
technique for collecting IPPM one-way active measurements, we hope to
create an environment where IPPM metrics may be collected across a
far broader mesh of Internet paths than is currently possible. One
particularly compelling vision is of widespread deployment of open
OWAMP servers that would make measurement of one-way delay as
commonplace as measurement of round-trip time using an ICMP-based
tool like ping.
Additional design goals of OWAMP include: being hard to detect and
manipulate, security, logical separation of control and test
functionality, and support for small test packets. (Being hard to
detect makes interference with measurements more difficult for
intermediaries in the middle of the network.)
OWAMP test traffic is hard to detect because it is simply a stream of
UDP packets from and to negotiated port numbers, with potentially
nothing static in the packets (size is negotiated, as well). OWAMP
also supports an encrypted mode that further obscures the traffic, at
the same time making it impossible to alter timestamps undetectably.
Security features include optional authentication and/or encryption
of control and test messages. These features may be useful to
prevent unauthorized access to results or man-in-the-middle attackers
who attempt to provide special treatment to OWAMP test streams or who
attempt to modify sender-generated timestamps to falsify test
results.
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
in this document are to be interpreted as described in [RFC2119].
Shalunov et al. [Page 3]

INTERNET-DRAFT One-way Active Measurement Protocol December 20041.1. Relationship of Test and Control Protocols
OWAMP actually consists of two inter-related protocols: OWAMP-Control
and OWAMP-Test. OWAMP-Control is used to initiate, start, and stop
test sessions and fetch their results, while OWAMP-Test is used to
exchange test packets between two measurement nodes.
Although OWAMP-Test may be used in conjunction with a control
protocol other than OWAMP-Control, the authors have deliberately
chosen to include both protocols in the same draft to encourage the
implementation and deployment of OWAMP-Control as a common
denominator control protocol for one-way active measurements. Having
a complete and open one-way active measurement solution that is
simple to implement and deploy is crucial to assuring a future in
which inter-domain one-way active measurement could become as
commonplace as ping. We neither anticipate nor recommend that
OWAMP-Control form the foundation of a general-purpose extensible
measurement and monitoring control protocol.
OWAMP-Control is designed to support the negotiation of one-way
active measurement sessions and results retrieval in a
straightforward manner. At session initiation, there is a negotiation
of sender and receiver addresses and port numbers, session start
time, session length, test packet size, the mean Poisson sampling
interval for the test stream, and some attributes of the very general
RFC 2330 notion of packet type, including packet size and per-hop
behavior (PHB) [RFC2474], which could be used to support the
measurement of one-way network characteristics across differentiated
services networks. Additionally, OWAMP-Control supports per-session
encryption and authentication for both test and control traffic,
measurement servers that can act as proxies for test stream
endpoints, and the exchange of a seed value for the pseudo-random
Poisson process that describes the test stream generated by the
sender.
We believe that OWAMP-Control can effectively support one-way active
measurement in a variety of environments, from publicly accessible
measurement beacons running on arbitrary hosts to network monitoring
deployments within private corporate networks. If integration with
Simple Network Management Protocol (SNMP) or proprietary network
management protocols is required, gateways may be created.
Shalunov et al. [Page 4]

INTERNET-DRAFT One-way Active Measurement Protocol December 20041.2. Logical Model
Several roles are logically separated to allow for broad flexibility
in use. Specifically, we define:
Session-Sender the sending endpoint of an OWAMP-Test session;
Session-Receiver the receiving endpoint of an OWAMP-Test session;
Server an end system that manages one or more OWAMP-Test
sessions, is capable of configuring per-session
state in session endpoints, and is capable of
returning the results of a test session;
Control-Client an end system that initiates requests for
OWAMP-Test sessions, triggers the start of a set
of sessions, and may trigger their termination; and
Fetch-Client an end system that initiates requests to fetch
the results of completed OWAMP-Test sessions.
One possible scenario of relationships between these roles is shown
below.
+----------------+ +------------------+
| Session-Sender |--OWAMP-Test-->| Session-Receiver |
+----------------+ +------------------+
^ ^
| |
| |
| |
| +----------------+<----------------+
| | Server |<-------+
| +----------------+ |
| ^ |
| | |
| OWAMP-Control OWAMP-Control
| | |
v v v
+----------------+ +-----------------+
| Control-Client | | Fetch-Client |
+----------------+ +-----------------+
(Unlabeled links in the figure are unspecified by this draft and may
be proprietary protocols.)
Different logical roles can be played by the same host. For example,
in the figure above, there could actually be only two hosts: one
Shalunov et al. [Page 5]

INTERNET-DRAFT One-way Active Measurement Protocol December 2004
playing the roles of Control-Client, Fetch-Client, and
Session-Sender, and the other playing the roles of Server and
Session-Receiver. This is shown below.
+-----------------+ +------------------+
| Control-Client |<--OWAMP-Control-->| Server |
| Fetch-Client | | |
| Session-Sender |---OWAMP-Test----->| Session-Receiver |
+-----------------+ +------------------+
Finally, because many Internet paths include segments that transport
IP over ATM, delay and loss measurements can include the effects of
ATM segmentation and reassembly (SAR). Consequently, OWAMP has been
designed to allow for small test packets that would fit inside the
payload of a single ATM cell (this is only achieved in
unauthenticated and encrypted modes).
2. Protocol Overview
As described above, OWAMP consists of two inter-related protocols:
OWAMP-Control and OWAMP-Test. The former is layered over TCP and is
used to initiate and control measurement sessions and to fetch their
results. The latter protocol is layered over UDP and is used to send
singleton measurement packets along the Internet path under test.
The initiator of the measurement session establishes a TCP connection
to a well-known port on the target point and this connection remains
open for the duration of the OWAMP-Test sessions. IANA will be
requested to allocate a well-known port number for OWAMP-Control
sessions. An OWAMP server SHOULD listen to this well-known port.
OWAMP-Control messages are transmitted only before OWAMP-Test
sessions are actually started and after they complete (with the
possible exception of an early Stop-Sessions message).
The OWAMP-Control and OWAMP-Test protocols support three modes of
operation: unauthenticated, authenticated, and encrypted. The
authenticated or encrypted modes require endpoints to possess a
shared secret.
All multi-octet quantities defined in this document are represented
as unsigned integers in network byte order unless specified
otherwise.
Shalunov et al. [Page 6]

INTERNET-DRAFT One-way Active Measurement Protocol December 20043. OWAMP-Control
Each type of OWAMP-Control message has a fixed length. The recipient
will know the full length of a message after examining the first 16
octets of it. No message is shorter than 16 octets.
An implementation SHOULD expunge unused state to prevent denial-of-
service attacks, or unbounded memory usage, on the server. For
example, if the full control message is not received within some
number of minutes after it is expected, the TCP connection associated
with the OWAMP-Control session SHOULD be dropped. In absence of
other considerations, 30 minutes seems like a reasonable upper bound.
3.1. Connection Setup
Before either a Control-Client or a Fetch-Client can issue commands
of a Server, it has to establish a connection to the server.
First, a client opens a TCP connection to the server on a well-known
port. The server responds with a server greeting:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Unused (12 octets) |
| |
|+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Modes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Challenge (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following Mode values are meaningful: 1 for unauthenticated, 2
for authenticated, and 4 for encrypted. The value of the Modes field
sent by the server is the bit-wise OR of the mode values that it is
willing to support during this session. Thus, the last three bits of
the Modes 32-bit value are used. The first 29 bits MUST be zero. A
client MUST ignore the values in the first 29 bits of the Modes
value. (This way, the bits are available for future protocol
extensions. This is the only intended extension mechanism.)
Challenge is a random sequence of octets generated by the server; it
is used subsequently by the client to prove possession of a shared
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
secret in a manner prescribed below.
If Modes value is zero, the server does not wish to communicate with
the client and MAY close the connection immediately. The client
SHOULD close the connection if it receives a greeting with Modes
equal to zero. The client MAY close the connection if the client's
desired mode is unavailable.
Otherwise, the client MUST respond with the following message:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Username (16 octets) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Token (32 octets) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Client-IV (16 octets) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Here Mode is the mode that the client chooses to use during this
OWAMP-Control session. It will also be used for all OWAMP-Test
sessions started under control of this OWAMP-Control session. In
Mode, one or zero bits MUST be set within last three bits. The first
29 bits of Mode MUST be zero. A server MUST ignore the values of the
first 29 bits. If zero Mode bits are set by the client, the client
indicates that it will not continue with the session; in this case,
the client and the server SHOULD close the TCP connection associated
with the OWAMP-Control session.
In unauthenticated mode, Username, Token, and Client-IV are unused.
Otherwise, Username is a 16-octet indicator that tells the server
which shared secret the client wishes to use to authenticate or
Shalunov et al. [Page 8]

INTERNET-DRAFT One-way Active Measurement Protocol December 2004
encrypt, while Token is the concatenation of a 16-octet challenge and
a 16-octet Session-key, encrypted using the AES (Advanced Encryption
Standard) [AES] in Cipher Block Chaining (CBC). Encryption MUST be
performed using an Initialization Vector (IV) of zero and a key value
that is the shared secret associated with Username. (Both the server
and the client use the same mappings from user names to secret keys.
The server, being prepared to conduct sessions with more than one
client, uses user names to choose the appropriate secret key; a
client would typically have different secret keys for different
servers. The situation is analogous to that of passwords, except
that secret keys, rather than having the low entropy typical of
passwords, are suitable for use as AES keys.)
The shared secret MUST be generated with sufficient entropy not to
reduce the security of the underlying cipher. Typical methods of its
generation might be from a random number generator [RFC1750] or from
the hash of a passphrase. If the shared secret is provided as a
passphrase (typical for the case of interactive tools) then the MD5
sum [RFC1321] of the passphrase (without possible newline
character(s) at the end of the passphrase) MUST be used as the key
for encryption by the client and decryption by the server (the
passphrase also MUST NOT contain newlines in the middle). This
ensures that a passphrase used to generate a secret in one
implementation will generate the same secret in another
implementation and the implementations will, therefore, be
interoperable.
Session-key and Client-IV are generated randomly by the client.
Session-key MUST be generated with sufficient entropy not to reduce
the security of the underlying cipher [RFC1750]. Client-IV merely
needs to be unique (i.e., it MUST never be repeated for different
sessions using the same secret key; a simple way to achieve that
without the use of cumbersome state is to generate the Client-IV
strings using a cryptographically secure pseudo-random number source:
if this is done, the first repetition is unlikely to occur before
2^64 sessions with the same secret key are conducted).
The server MUST respond with the following message:
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MBZ (15 octets) |
| |
| +-+-+-+-+-+-+-+-+
| | Accept |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Server-IV (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Uptime (Timestamp) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IZP (8 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MBZ 15-octet part MUST be zero. The client MUST ignore its
value. MBZ (MUST be zero) fields here and hereafter have the same
semantics: the party that sends the message MUST set the field to a
string of zero bits; the party that interprets the message MUST
ignore the value. (This way the field could be used for future
extensions.)
Server-IV is generated randomly by the server. In unauthenticated
mode, Server-IV is unused.
The Accept field indicates the server's willingness to continue
communication. A zero value in the Accept field means that the
server accepts the authentication and is willing to conduct further
transactions. Non-zero values indicate that the server does not
accept the authentication or, for some other reason, is not willing
to conduct further transactions in this OWAMP-Control session. The
full list of available Accept values is described in Section 3.3,
``Values of the Accept Field''.
If a negative (non-zero) response is sent, the server MAY and the
client SHOULD close the connection after this message.
Uptime is a timestamp representing the time when the current
instantiation of the server started operating. (For example, in a
multi-user general purpose operating system (OS), it could be the
time when the server process was started.) If Accept is non-zero,
Uptime SHOULD be set to a string of zeros. In authenticated and
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
encrypted modes, Uptime is encrypted as described in the next
section, unless Accept is non-zero. (Authenticated and encrypted mode
cannot be entered unless the control connection can be initialized.)
Timestamp format is described in Section 4.1.2. The same
instantiation of the server SHOULD report the same exact Uptime value
to each client in each session.
Integrity Zero Padding (IZP) is treated the same way as IZP in the
next section and beyond.
The previous transactions constitute connection setup.
3.2. Integrity Zero Padding (IZP)
IZP MUST be all zeros in all messages that use IZP. The recipient of
a message where IZP is not zero MUST reject the message, as it is an
indication of tampering with the content of the message by an
intermediary (or brokenness). If the message is part of
OWAMP-Control, the session MUST be terminated and results
invalidated. If the message is part of OWAMP-Test, it MUST be
silently ignored. This will ensure data integrity. In
unauthenticated mode, IZP is nothing more than a simple check. In
authenticated and encrypted modes, however, it ensures, in
conjunction with properties of CBC chaining mode, that everything
received before was not tampered with. For this reason, it is
important to check the IZP field as soon as possible, so that bad
data doesn't get propagated.
3.3. Values of the Accept Field
Accept values are used throughout the OWAMP-Control protocol to
communicate the server response to client requests. The full set of
valid Accept field values are:
0 OK.
1 Failure, reason unspecified (catch-all).
2 Internal error.
3 Some aspect of request is not supported.
4 Cannot perform request due to permanent resource limitations.
5 Cannot perform request due to temporary resource limitations.
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
All other values are reserved. The sender of the message MAY use the
value of 1 for all non-zero Accept values. A message sender SHOULD
use the correct Accept value if it is going to use other values. The
message receiver MUST interpret all values of Accept other than these
reserved values as 1. This way, other values are available for
future extensions.
3.4. OWAMP-Control Commands
In authenticated or encrypted mode (which are identical as far as
OWAMP-Control is concerned, and only differ in OWAMP-Test) all
further communications are encrypted with the Session-key, using CBC
mode. The client encrypts its stream using Client-IV. The server
encrypts its stream using Server-IV.
The following commands are available for the client: Request-Session,
Start-Sessions, Stop-Sessions, and Fetch-Session. The command
Stop-Sessions is available to both the client and the server. (The
server can also send other messages in response to commands it
receives.)
After the client sends the Start-Sessions command and until it both
sends and receives (in an unspecified order) the Stop-Sessions
command, it is said to be conducting active measurements. Similarly,
the server is said to be conducting active measurements after it
receives the Start-Sessions command and until it both sends and
receives (in an unspecified order) the Stop-Sessions command.
While conducting active measurements, the only command available is
Stop-Sessions.
These commands are described in detail below.
3.5. Creating Test Sessions
Individual one-way active measurement sessions are established using
a simple request/response protocol. An OWAMP client MAY issue zero or
more Request-Session messages to an OWAMP server, which MUST respond
to each with an Accept-Session message. An Accept-Session message
MAY refuse a request.
The format of Request-Session message is as follows:
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
descriptions (the number of schedule slots is specified in the
`Number of Schedule Slots' field above):
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Slot Type | |
+-+-+-+-+-+-+-+-+ MBZ (15 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Slot Parameter (Timestamp) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are immediately followed by IZP:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IZP (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All these messages comprise one logical message: the Request-Session
command.
Above, the first octet (1) indicates that this is Request-Session
command.
IPVN is the IP version numbers for Sender and Receiver. When the IP
version number is 4, 12 octets follow the 4-octet IPv4 address stored
in Sender Address and Receiver Address. These octets MUST be set to
zero by the client and MUST be ignored by the server. Currently
meaningful IPVN values are 4 and 6.
Conf-Sender and Conf-Receiver MUST be set to 0 or 1 by the client.
The server MUST interpret any non-zero value as 1. If the value is
1, the server is being asked to configure the corresponding agent
(sender or receiver). In this case, the corresponding Port value
SHOULD be disregarded by the server. At least one of Conf-Sender and
Conf-Receiver MUST be 1. (Both can be set, in which case the server
is being asked to perform a session between two hosts it can
configure.)
Number of Schedule Slots, as mentioned before, specifies the number
of slot records that go between the two blocks of IZP. It is used by
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
the sender to determine when to send test packets (see next section).
Number of Packets is the number of active measurement packets to be
sent during this OWAMP-Test session (note that either the server or
the client can abort the session early).
If Conf-Sender is not set, Sender Port is the UDP port from which
OWAMP-Test packets will be sent. If Conf-Receiver is not set,
Receiver Port is the UDP port OWAMP-Test to which packets are
requested to be sent.
The Sender Address and Receiver Address fields contain, respectively,
the sender and receiver addresses of the end points of the Internet
path over which an OWAMP test session is requested.
SID is the session identifier. It can be used in later sessions as
an argument for the Fetch-Session command. It is meaningful only if
Conf-Receiver is 0. This way, the SID is always generated by the
receiving side. See the end of the section for information on how
the SID is generated.
Padding length is the number of octets to be appended to the normal
OWAMP-Test packet (see more on padding in discussion of OWAMP-Test).
Start Time is the time when the session is to be started (but not
before Start-Sessions command is issued). This timestamp is in the
same format as OWAMP-Test timestamps.
Timeout (or a loss threshold) is an interval of time (expressed as a
timestamp). A packet belonging to the test session that is being set
up by the current Request-Session command will be considered lost if
it is not received during Timeout seconds after it is sent.
Type-P Descriptor covers only a subset of (very large) Type-P space.
If the first two bits of the Type-P Descriptor are 00, then
subsequent six bits specify the requested Differentiated Services
Codepoint (DSCP) value of sent OWAMP-Test packets, as defined in
RFC 2474. If the first two bits of Type-P descriptor are 01, then
the subsequent 16 bits specify the requested PHB Identification Code
(PHB ID), as defined in RFC 2836.
Therefore, the value of all zeros specifies the default best-effort
service.
If Conf-Sender is set, the Type-P Descriptor is to be used to
configure the sender to send packets according to its value. If
Conf-Sender is not set, the Type-P Descriptor is a declaration of how
the sender will be configured.
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
If Conf-Sender is set and the server does not recognize the Type-P
Descriptor, or it cannot or does not wish to set the corresponding
attributes on OWAMP-Test packets, it SHOULD reject the session
request. If Conf-Sender is not set, the server SHOULD accept or
reject the session paying no attention to the value of the Type-P
Descriptor.
To each Request-Session message, an OWAMP server MUST respond with an
Accept-Session message:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Accept | MBZ | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
| SID (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IZP (12 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In this message, zero in the Accept field means that the server is
willing to conduct the session. A non-zero value indicates rejection
of the request. The full list of available Accept values is
described in Section 3.3, ``Values of the Accept Field''.
If the server rejects a Request-Session message, it SHOULD not close
the TCP connection. The client MAY close it if it receives negative
response to the Request-Session message.
The meaning of Port in the response depends on the values of
Conf-Sender and Conf-Receiver in the query that solicited the
response. If both were set, the Port field is unused. If only
Conf-Sender was set, Port is the port from which to expect OWAMP-Test
packets. If only Conf-Receiver was set, Port is the port to which
OWAMP-Test packets are sent.
If only Conf-Sender was set, the SID field in the response is unused.
Otherwise, SID is a unique server-generated session identifier. It
can be used later as handle to fetch the results of a session.
SIDs SHOULD be constructed by concatenation of the 4-octet IPv4 IP
number belonging to the generating machine, an 8-octet timestamp, and
a 4-octet random value. To reduce the probability of collisions, if
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
the generating machine has any IPv4 addresses (with the exception of
loopback), one of them SHOULD be used for SID generation, even if all
communication is IPv6-based. If it has no IPv4 addresses at all, the
last four octets of an IPv6 address MAY be used instead. Note that
SID is always chosen by the receiver. If truly random values are not
available, it is important that the SID be made unpredictable, as
knowledge of the SID might be used for access control.
3.6. Send Schedules
The sender and the receiver both need to know the same send schedule.
This way, when packets are lost, the receiver knows when they were
supposed to be sent. It is desirable to compress common schedules
and still to be able to use an arbitrary one for the test sessions.
In many cases, the schedule will consist of repeated sequences of
packets: this way, the sequence performs some test, and the test is
repeated a number of times to gather statistics.
To implement this, we have a schedule with a given number of slots.
Each slot has a type and a parameter. Two types are supported:
exponentially distributed pseudo-random quantity (denoted by a code
of 0) and a fixed quantity (denoted by a code of 1). The parameter
is expressed as a timestamp and specifies a time interval. For a
type 0 slot (exponentially distributed pseudo-random quantity) this
interval is the mean value (or 1/lambda if the distribution density
function is expressed as lambda*exp(-lambda*x) for positive values of
x). For a type 1 (fixed quantity) slot, the parameter is the delay
itself. The sender starts with the beginning of the schedule, and
executes the instructions in the slots: for a slot of type 0, wait an
exponentially distributed time with a mean of the specified parameter
and then send a test packet (and proceed to the next slot); for a
slot of type 1, wait the specified time and send a test packet (and
proceed to the next slot). The schedule is circular: when there are
no more slots, the sender returns to the first slot.
The sender and the receiver need to be able to reproducibly execute
the entire schedule (so, if a packet is lost, the receiver can still
attach a send timestamp to it). Slots of type 1 are trivial to
reproducibly execute. To reproducibly execute slots of type 0, we
need to be able to generate pseudo-random exponentially distributed
quantities in a reproducible manner. The way this is accomplished is
discussed later.
Using this mechanism one can easily specify common testing scenarios.
Some examples include:
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
command.
Above, the first octet (3) indicates that this is the Stop-Sessions
command.
Non-zero Accept values indicate a failure of some sort. Zero values
indicate normal (but possibly premature) completion. The full list
of available Accept values is described in Section 3.3, ``Values of
the Accept Field''.
If Accept had a non-zero value (from either party), results of all
OWAMP-Test sessions spawned by this OWAMP-Control session SHOULD be
considered invalid, even if a Fetch-Session with SID from this
session works for a different OWAMP-Control session. If Accept was
not transmitted at all (for whatever reason, including the TCP
connection used for OWAMP-Control breaking), the results of all
OWAMP-Test sessions spawned by this OWAMP-control session MAY be
considered invalid.
Number of Sessions indicates the number of session description
records that immediately follow the Stop-Sessions header.
Number of Sessions MUST contain the number of send sessions started
by the local side of the control connection that have not been
previously terminated by a Stop-Sessions command (i.e., the
Control-Client MUST account for each accepted Request-Session where
Conf-Receiver was set; the Control-Server MUST account for each
accepted Request-Session where Conf-Sender was set). If the
Stop-Sessions message does not account for exactly the send sessions
controlled by that side, then it is to be considered invalid and the
connection SHOULD be closed and any results obtained considered
invalid.
Each session description record represents one OWAMP-Test session.
SID is the session identifier (SID) used to indicate which send
session is being described.
Next Seqno indicates the next sequence number that would have been
sent from this send session. For completed sessions, this will equal
NumPackets from the Request-Session.
Number of Skip Ranges indicates the number of holes that actually
occurred in the sending process. This is a range of packets that were
never actually sent by the sending process. For example, if a send
session is started too late for the first 10 packets to be sent and
this is the only hole in the schedule, then ``Number of Skip Ranges''
would be 1. The single Skip Range description will have First Seqno
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Skipped equal to 0 and Last Seqno Skipped equal to 9. This is
described further in the ``Sender Behavior'' section.
If the OWAMP-Control connection breaks when the Stop-Sessions command
is sent, the receiver MAY not completely invalidate the session
results. It MUST discard all record of packets that follow (in other
words, have greater sequence number than) the last packet that was
actually received before before any lost packet records. This will
help differentiate between packet losses that occurred in the network
and packets the sending process may have never sent.
If a receiver of an OWAMP-Test session learns, through an OWAMP-
Control Stop-Sessions message, that the OWAMP-Test sender's last
sequence number is lower than any sequence number actually received,
the results of the complete OWAMP-Test session MUST be invalidated.
A receiver of an OWAMP-Test session, upon receipt of an OWAMP-Control
Stop-Sessions command, MUST discard any packet records -- including
lost packet records -- with a (computed) send time that falls between
the current time minus Timeout and the current time. This ensures
statistical consistency for the measurement of loss and duplicates in
the event that the Timeout is greater than the time it takes for the
Stop-Sessions command to take place.
To effect complete sessions, each side of the control connection
SHOULD wait until all sessions are complete before sending the
Stop-Sessions message. The completed time of each sessions is
determined as Timeout after the scheduled time for the last sequence
number. Endpoints MAY add a small increment to the computed
completed time for send endpoints to ensure the Stop-Sessions message
reaches the receiver endpoint after Timeout.
To effect a premature stop of sessions, the party that initiates this
command MUST stop its OWAMP-Test send streams to send the Session
Packets Sent values before sending this command. That party SHOULD
wait until receiving the response Stop-Sessions message before
stopping the receiver streams so that it can use the values from the
received Stop-Sessions message to validate the data.
3.9. Fetch-Session
The format of this client command is as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | |
+-+-+-+-+-+-+-+-+ |
| MBZ (7 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Begin Seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| End Seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| SID (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IZP (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Begin Seq is the sequence number of the first requested packet. End
Seq is the sequence number of the last requested packet. If Begin
Seq is all zeros and End Seq is all ones, complete session is said to
be requested.
If a complete session is requested and the session is still in
progress, or has terminated in any way other than normal, the request
to fetch session results MUST be denied. If an incomplete session is
requested, all packets received so far that fall into the requested
range SHOULD be returned. Note that, since no commands can be issued
between Start-Sessions and Stop-Sessions, incomplete requests can
only happen on a different OWAMP-Control connection (from the same or
different host as Control-Client).
The server MUST respond with a Fetch-Ack message. The format of this
server response is as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Accept | Finished | MBZ (2 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Seqno |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Skip Ranges |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Records |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IZP (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Again, non-zero in the Accept field means a rejection of command.
The server MUST specify zero for all remaining fields if Accept is
non-zero. The client MUST ignore all remaining fields (except for the
IZP) if Accept is non-zero. The full list of available Accept values
is described in Section 3.3, ``Values of the Accept Field''.
Finished is non-zero if the OWAMP-Test session has terminated.
Next Seqno indicates the next sequence number that would have been
sent from this send session. For completed sessions, this will equal
NumPackets from the Request-Session. This information is only
available if the session has terminated. If Finished is zero, then
Next Seqno MUST be set to zero by the server.
Number of Skip Ranges indicates the number of holes that actually
occurred in the sending process. This information is only available
if the session has terminated. If Finished is zero, then Skip Ranges
MUST be set to zero by the server.
Number of Records is the number of packet records that fall within
the requested range. This number might be less than the Number of
Packets in the reproduction of the Request-Session command because of
a session that ended prematurely or it might be greater because of
duplicates.
If Accept was non-zero, this concludes the response to the Fetch-
Session message. If Accept was 0, the server then MUST immediately
send the OWAMP-Test session data in question.
The OWAMP-Test session data consists of the following (concatenated):
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+ A reproduction of the Request-Session command that was used to
start the session; it is modified so that actual sender and
receiver port numbers that were used by the OWAMP-Test session
always appear in the reproduction.
+ Zero or more (as specified) Skip Range descriptions. The last
(possibly full, possibly incomplete) block (16 octets) of Skip
Range descriptions is padded with zeros if necessary. (These
zeros are simple padding and should be distinguished from the 16
octets of IZP that follow.)
+ 16 octets of IZP.
+ Zero or more (as specified) packet records. The last (possibly
full, possibly incomplete) block (16 octets) of data is padded
with zeros if necessary. (These zeros are simple padding and
should be distinguished from the 16 octets of IZP that follow.)
+ 16 octets of IZP.
Skip Range descriptions are simply two sequence numbers that,
together, indicate a range of packets that were not sent:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| First Seqno Skipped |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Seqno Skipped |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Skip Range descriptions should be sent out in order, as sorted by
First Seqno. If any Skip Ranges overlap, or are out of order, the
session data is to be considered invalid and the connection SHOULD be
closed and any results obtained considered invalid.
Each packet record is 25 octets, and includes 4 octets of sequence
number, 8 octets of send timestamp, 2 octets of send timestamp error
estimate, 8 octets of receive timestamp, 2 octets of receive
timestamp error estimate, and 1 octet of Time To Live (TTL), or Hop
Limit in IPv6:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00| Seq Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
04| Send Error Estimate | Receive Error Estimate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
08| Send Timestamp |
12| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16| Receive Timestamp |
20| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
24| TTL |
+-+-+-+-+-+-+-+-+
Packet records are sent out in the same order the actual packets were
received. Therefore, the data is in arrival order.
Note that lost packets (if any losses were detected during the
OWAMP-Test session) MUST appear in the sequence of packets. They can
appear either at the point when the loss was detected or at any later
point. Lost packet records are distinguished as follows:
+ A send timestamp filled with the presumed send time (as computed
by the send schedule).
+ A send error estimate filled with Multiplier=1, Scale=64, and S=0
(see the OWAMP-Test description for definition of these quantities
and explanation of timestamp format and error estimate format).
+ A normal receive error estimate as determined by the error of the
clock being used to declare the packet lost. (It is declared lost
if it is not received by the Timeout after the presumed send time,
as determined by the receiver's clock.)
+ A receive timestamp consisting of all zero bits.
+ A TTL value of 255.
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This section describes OWAMP-Test protocol. It runs over UDP using
sender and receiver IP and port numbers negotiated during the
Request-Session exchange.
As with OWAMP-Control, OWAMP-Test has three modes: unauthenticated,
authenticated, and encrypted. All OWAMP-Test sessions that are
spawned by an OWAMP-Control session inherit its mode.
OWAMP-Control client, OWAMP-Control server, OWAMP-Test sender, and
OWAMP-Test receiver can potentially all be different machines. (In a
typical case, we expect that there will be only two machines.)
4.1. Sender Behavior4.1.1. Packet Timings
Send schedules based on slots, described previously, in conjunction
with scheduled session start time, enable the sender and the receiver
to compute the same exact packet sending schedule independently of
each other. These sending schedules are independent for different
OWAMP-Test sessions, even if they are governed by the same
OWAMP-Control session.
Consider any OWAMP-Test session. Once Start-Sessions exchange is
complete, the sender is ready to start sending packets. Under normal
OWAMP use circumstances, the time to send the first packet is in the
near future (perhaps a fraction of a second away). The sender SHOULD
send packets as close as possible to their scheduled time, with the
following exception: if the scheduled time to send is in the past,
and separated from the present by more than Timeout time, the sender
MUST NOT send the packet. (Indeed, such a packet would be considered
lost by the receiver anyway.) The sender MUST keep track of which
packets it does not send. It will use this to tell the receiver what
packets were not sent by setting Skip Ranges in the Stop-Sessions
message from the sender to the receiver upon completion of the test.
The Skip Ranges are also sent to a Fetch-Client as part of the
session data results. These holes in the sending schedule can happen
if a time in the past was specified in the Request-Session command,
or if the Start-Sessions exchange took unexpectedly long, or if the
sender could not start serving the OWAMP-Test session on time due to
internal scheduling problems of the OS. Packets in the past, but
separated from the present by less than Timeout value, SHOULD be sent
as quickly as possible. With normal test rates and timeout values,
the number of packets in such a burst is limited. Nevertheless,
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
(e.g., the bit should be set if GPS hardware is used and it indicates
that it has acquired current position and time or if NTP is used and
it indicates that it has synchronized to an external source, which
includes stratum 0 source, etc.); if there is no notion of external
synchronization for the time source, the bit SHOULD NOT be set. The
next bit has the same semantics as MBZ fields elsewhere: it MUST be
set to zero by the sender and ignored by everyone else. The next six
bits, Scale, form an unsigned integer; Multiplier is an unsigned
integer as well. They are interpreted as follows: the error estimate
is equal to Multiplier*2^(-32)*2^Scale (in seconds). [Notation
clarification: 2^Scale is two to the power of Scale.] Multiplier
MUST NOT be set to zero. If Multiplier is zero, the packet SHOULD be
considered corrupt and discarded.
Sequence numbers start with zero and are incremented by one for each
subsequent packet.
The minimum data segment length is, therefore, 14 octets in
unauthenticated mode, and 32 octets in both authenticated mode and
encrypted modes.
The OWAMP-Test packet layout is the same in authenticated and
encrypted modes. The encryption operations are, however, different.
The difference is that in encrypted mode both the sequence number and
the timestamp are encrypted to provide maximum data integrity
protection while in authenticated mode the sequence number is
encrypted and the timestamp is sent in clear text. Sending the
timestamp in clear text in authenticated mode allows one to reduce
the time between when a timestamp is obtained by a sender and when
the packet is shipped out. In encrypted mode, the sender has to
fetch the timestamp, encrypt it, and send it; in authenticated mode,
the middle step is removed, potentially improving accuracy (the
sequence number can be encrypted before the timestamp is fetched).
In authenticated mode, the first block (16 octets) of each packet is
encrypted using AES Electronic Cookbook (ECB) mode.
The key to use is obtained as follows: the 16-octet session
identifier (SID) is encrypted with the same session key as is used
for the corresponding OWAMP-Control session (where it is used in a
different chaining mode); this is a single-block ECB encryption; its
result is the key to use in encrypting (and decrypting) the packets
of the particular OWAMP-Test session.
ECB mode used for encrypting the first block of OWAMP-Test packets in
authenticated mode does not involve any actual chaining; this way,
lost, duplicated, or reordered packets do not cause problems with
deciphering any packet in an OWAMP-Test session.
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In encrypted mode, the first two blocks (32 octets) are encrypted
using AES CBC mode. The key to use is obtained in the same way as
the key for authenticated mode. Each OWAMP-Test packet is encrypted
as a separate stream, with just one chaining operation; chaining does
not span multiple packets so that lost, duplicated, or reordered
packets do not cause problems. The initialization vector for the CBC
encryption is a string of zeros.
Implementation note: Naturally, the key schedule for each OWAMP-Test
session need only be set up once per session, not once per packet.
In unauthenticated mode, no encryption is applied.
Packet Padding in OWAMP-Test SHOULD be pseudo-random (it MUST be
generated independently of any other pseudo-random numbers mentioned
in this document). However, implementations MUST provide a
configuration parameter, an option, or a different means of making
Packet Padding consist of all zeros.
The time elapsed between packets is computed according to the slot
schedule as mentioned in Request-Session command description. At
that point, we skipped over the issue of computing exponentially
distributed pseudo-random numbers in a reproducible fashion. It is
discussed later in a separate section.
4.2. Receiver Behavior
The receiver knows when the sender will send packets. The following
parameter is defined: Timeout (from Request-Session). Packets that
are delayed by more than Timeout are considered lost (or `as good as
lost'). Note that there is never an actual assurance of loss by the
network: a `lost' packet might still be delivered at any time. The
original specification for IPv4 required that packets be delivered
within TTL seconds or never (with TTL having a maximum value of 255).
To the best of the authors' knowledge, this requirement was never
actually implemented (and, of course, only a complete and universal
implementation would ensure that packets do not travel for longer
than TTL seconds). In fact, in IPv6, the name of this field has
actually been changed to Hop Limit. Further, IPv4 specification
makes no claims about the time it takes the packet to traverse the
last link of the path.
The choice of a reasonable value of Timeout is a problem faced by a
user of OWAMP protocol, not by an implementor. A value such as two
minutes is very safe. Note that certain applications (such as
interactive `one-way ping') might wish to obtain the data faster than
that.
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
As packets are received,
+ Timestamp the received packet.
+ In authenticated or encrypted mode, decrypt the first block (16
octets) of the packet body.
+ Store the packet sequence number, send time, receive time, and the
TTL for IPv4 (or Hop Limit for IPv6) from the packet IP header for
the results to be transferred.
+ Packets not received within the Timeout are considered lost. They
are recorded with their true sequence number, presumed send time,
receive time consisting of a string of zero bits, and TTL (or Hop
Limit) of 255.
Implementations SHOULD fetch the TTL/Hop Limit value from the IP
header of the packet. If an implementation does not fetch the actual
TTL value (the only good reason to not do so is inability to access
the TTL field of arriving packets), it MUST record the TTL value as
255.
Packets that are actually received are recorded in the order of
arrival. Lost packet records serve as indications of the send times
of lost packets. They SHOULD be placed either at the point where the
receiver learns about the loss or at any later point; in particular,
one MAY place all the records that correspond to lost packets at the
very end.
Packets that have send time in the future MUST be recorded normally,
without changing their send timestamp, unless they have to be
discarded. (Send timestamps in the future would normally indicate
clocks that differ by more than the delay. Some data -- such as
jitter -- can be extracted even without knowledge of time difference.
For other kinds of data, the adjustment is best handled by the data
consumer on the basis of the complete information in a measurement
session, as well as, possibly, external data.)
Packets with a sequence number that was already observed (duplicate
packets) MUST be recorded normally. (Duplicate packets are sometimes
introduced by IP networks. The protocol has to be able to measure
duplication.)
If any of the following is true, the packet MUST be discarded:
+ Send timestamp is more than Timeout in the past or in the future.
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
+ Send timestamp differs by more than Timeout from the time when the
packet should have been sent according to its sequence number.
+ In authenticated or encrypted mode, any of the bits of zero
padding inside the first 16 octets of packet body is non-zero.
5. Computing Exponentially Distributed Pseudo-Random Numbers
Here we describe the way exponential random quantities used in the
protocol are generated. While there is a fair number of algorithms
for generating exponential random variables, most of them rely on
having logarithmic function as a primitive, resulting in potentially
different values, depending on the particular implementation of the
math library. We use algorithm 3.4.1.S in [KNUTH], which is free
of the above-mentioned problem, and guarantees the same output on any
implementation. The algorithm belongs to the ziggurat family
developed in the 1970s by G. Marsaglia, M. Sibuya and J. H. Ahrens
[ZIGG]. It replaces the use of logarithmic function by clever bit
manipulation, still producing the exponential variates on output.
5.1. High-Level Description of the Algorithm
For ease of exposition, the algorithm is first described with all
arithmetic operations being interpreted in their natural sense.
Later, exact details on data types, arithmetic, and generation of the
uniform random variates used by the algorithm are given. It is an
almost verbatim quotation from [KNUTH], p.133.
Algorithm S: Given a real positive number 'mu', produce an
exponential random variate with mean 'mu'.
First, the constants
Q[k] = (ln2)/(1!) + (ln2)^2/(2!) + ... + (ln2)^k/(k!), 1 <= k <= 11
are computed in advance. The exact values which MUST be used by all
implementations are given in the next section. This is necessary to
insure that exactly the same pseudo-random sequences are produced by
all implementations.
S1. [Get U and shift.] Generate a 32-bit uniform random binary
fraction
U = (.b0 b1 b2 ... b31) [note the binary point]
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
Locate the first zero bit b_j, and shift off the leading (j+1) bits,
setting U <- (.b_{j+1} ... b31)
Note: In the rare case that the zero has not been found, it is
prescribed that the algorithm return (mu*32*ln2).
S2. [Immediate acceptance?] If U < ln2, set X <- mu*(j*ln2 + U) and
terminate the algorithm. (Note that Q[1] = ln2.)
S3. [Minimize.] Find the least k >= 2 such that U < Q[k]. Generate k
new uniform random binary fractions U1,...,Uk and set V <-
min(U1,...,Uk).
S4. [Deliver the answer.] Set X <- mu*(j + V)*ln2.
5.2. Data Types, Representation, and Arithmetic
The high-level algorithm operates on real numbers -- typically
represented as floating point numbers. This specification prescribes
that unsigned 64-bit integers be used instead.
u_int64_t integers are interpreted as real numbers by placing the
decimal point after the first 32 bits. In other words, conceptually,
the interpretation is given by the map:
u_int64_t u;
u |--> (double)u / (2**32)
The algorithm produces a sequence of such u_int64_t integers that,
for any given value of SID, is guaranteed to be the same on any
implementation.
We specify that the u_int64_t representations of the first 11 values
of the Q array in the high-level algorithm MUST be as follows:
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
#1 0xB17217F8,
#2 0xEEF193F7,
#3 0xFD271862,
#4 0xFF9D6DD0,
#5 0xFFF4CFD0,
#6 0xFFFEE819,
#7 0xFFFFE7FF,
#8 0xFFFFFE2B,
#9 0xFFFFFFE0,
#10 0xFFFFFFFE,
#11 0xFFFFFFFF
For example, Q[1] = ln2 is indeed approximated by 0xB17217F8/(2**32)
= 0.693147180601954; for j > 11, Q[j] is 0xFFFFFFFF.
Small integer j in the high-level algorithm is represented as
u_int64_t value j * (2**32).
Operation of addition is done as usual on u_int64_t numbers; however,
the operation of multiplication in the high-level algorithm should be
replaced by
(u, v) |---> (u * v) >> 32.
Implementations MUST compute the product (u * v) exactly. For
example, a fragment of unsigned 128-bit arithmetic can be implemented
for this purpose (see sample implementation below).
5.3. Uniform Random Quantities
The procedure for obtaining a sequence of 32-bit random numbers (such
as U in algorithm S) relies on using AES encryption in counter mode.
To describe the exact working of the algorithm, we introduce two
primitives from Rijndael. Their prototypes and specification are
given below, and they are assumed to be provided by the supporting
Rijndael implementation, such as [RIJN].
+ A function that initializes a Rijndael key with bytes from seed
(the SID will be used as the seed):
void KeyInit(unsigned char seed[16]);
+ A function that encrypts the 16-octet block inblock with the
specified key, returning a 16-octet encrypted block. Here
keyInstance is an opaque type used to represent Rijndael keys:
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
void BlockEncrypt(keyInstance key, unsigned char inblock[16]);
Algorithm Unif: given a 16-octet quantity seed, produce a sequence of
unsigned 32-bit pseudo-random uniformly distributed integers. In
OWAMP, the SID (session ID) from Control protocol plays the role of
seed.
U1. [Initialize Rijndael key] key <- KeyInit(seed) [Initialize an
unsigned 16-octet (network byte order) counter] c <- 0 U2. [Need
more random bytes?] Set i <- c mod 4. If (i == 0) set s <-
BlockEncrypt(key, c)
U3. [Increment the counter as unsigned 16-octet quantity] c <- c + 1
U4. [Do output] Output the i_th quartet of octets from s starting
from high-order octets, converted to native byte order and
represented as OWPNum64 value (as in 3.b).
U5. [Loop] Go to step U2.
6. Security Considerations6.1. Introduction
The goal of authenticated mode to let one passphrase-protect the
service provided by a particular OWAMP-Control server. One can
imagine a variety of circumstances where this could be useful.
Authenticated mode is designed to prohibit theft of service.
An additional design objective of the authenticated mode was to make
it impossible for an attacker who cannot read traffic between OWAMP-
Test sender and receiver to tamper with test results in a fashion
that affects the measurements, but not other traffic.
The goal of encrypted mode is quite different: to make it hard for a
party in the middle of the network to make results look `better' than
they should be. This is especially true if one of client and server
does not coincide with either sender or receiver.
Encryption of OWAMP-Control using AES CBC mode with blocks of zeros
after each message aims to achieve two goals: (i) to provide secrecy
of exchange; (ii) to provide authentication of each message.
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INTERNET-DRAFT One-way Active Measurement Protocol December 20046.2. Preventing Third-Party Denial of Service
OWAMP-Test sessions directed at an unsuspecting party could be used
for denial of service (DoS) attacks. In unauthenticated mode,
servers SHOULD limit receivers to hosts they control or to the OWAMP-
Control client.
6.3. Covert Information Channels
OWAMP-Test sessions could be used as covert channels of information.
Environments that are worried about covert channels should take this
into consideration.
6.4. Requirement to Include AES in Implementations
Notice that AES, in counter mode, is used for pseudo-random number
generation, so implementation of AES MUST be included, even in a
server that only supports unauthenticated mode.
6.5. Resource Use Limitations
An OWAMP server can consume resources of various kinds. The two most
important kinds of resources are network capacity and memory (primary
or secondary) for storing test results.
Any implementation of OWAMP server MUST include technical mechanisms
to limit the use of network capacity and memory. Mechanisms for
managing the resources consumed by unauthenticated users and users
authenticated with a username and passphrase SHOULD be separate. The
default configuration of an implementation MUST enable these
mechanisms and set the resource use limits to conservatively low
values.
One way to design the resource limitation mechanisms is as follows:
assign each session to a user class. User classes are partially
ordered with ``includes'' relation, with one class (``all users'')
that is always present and that includes any other class. The
assignment of a session to a user class can be based on the presence
of authentication of the session, the user name, IP address range,
time of day, and, perhaps, other factors. Each user class would have
a limit for usage of network capacity (specified in units of
bit/second) and memory for storing test results (specified in units
of octets). Along with the limits for resource use, current use
would be tracked by the server. When a session is requested by a
user in a specific user class, the resources needed for this session
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are computed: the average network capacity use (based on the sending
schedule) and the maximum memory use (based on the number of packets
and number of octets each packet would need to be stored internally
-- note that outgoing sessions would not require any memory use).
These resource use numbers are added to the current resource use
numbers for the given user class; if such addition would take the
resource use outside of the limits for the given user class, the
session is rejected. When resources are reclaimed, corresponding
measures are subtracted from the current use. Network capacity is
reclaimed as soon as the session ends. Memory is reclaimed when the
data is deleted. For unauthenticated sessions, memory consumed by an
OWAMP-Test session SHOULD be reclaimed after the OWAMP-Control
connection that initiated the session is closed (gracefully or
otherwise). For authenticated sessions, the administrator who
configures the service should be able to decide the exact policy, but
useful policy mechanisms that MAY be implemented are the ability to
automatically reclaim memory when the data is retrieved and the
ability to reclaim memory after a certain configurable (based on user
class) period of time passes after the OWAMP-Test session terminates.
6.6. Use of Cryptographic Primitives in OWAMP
At an early stage in designing the protocol, we considered using
Transport Layer Security (TLS) [RFC2246, RFC3546] and IPsec [RFC2401]
as cryptographic security mechanisms for OWAMP. The disadvantages of
those are as follows (not an exhaustive list):
Regarding TLS:
+ While TLS could be used to secure TCP-based OWAMP-Control, but
difficult to use to secure UDP-based OWAMP-Test: OWAMP-Test
packets, if lost, are not resent, so packets have to be
(optionally) encrypted and authenticated while retaining
individual usability. Stream-based TLS is not conducive of this.
+ Dealing with streams, does not authenticate individual messages
(even in OWAMP-Control). The easiest way out would be to add some
known-format padding to each message and verify that the format of
the padding is intact before using the message. The solution
would thus lose some of its appeal (``just use TLS''); it would
also be much more difficult to evaluate the security of this
scheme with the various modes and options of TLS -- it would
almost certainly not be secure with all. The capacity of an
attacker to replace parts of messages (namely, the end) with
random garbage could have serious security implications and would
need to be analyzed carefully: suppose, for example, that a
parameter that is used in some form to control the rate were
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replaced by random garbage -- chances are the result (an unsigned
integer) would be quite large.
+ Dependent on the mode of use, one can end up with a requirement
for certificates for all users and a PKI. Even if one is to
accept that PKI is desirable, there just isn't a usable one today.
+ TLS requires a fairly large implementation. OpenSSL, for example,
is larger than our implementation of OWAMP as a whole. This can
matter for embedded implementations.
Regarding IPsec:
+ What we now call authenticated mode would not be possible (in
IPsec you can't authenticate part of a packet).
+ The deployment paths of IPsec and OWAMP could be separate if OWAMP
does not depend on IPsec. After nine years of IPsec, only 0.05%
of traffic on an advanced backbone network such as Abilene uses
IPsec (for comparison purposes with encryption above layer 4, SSH
use is at 2-4% and HTTPS use is at 0.2-0.6%). It is desirable to
be able to deploy OWAMP on as large of a number of different
platforms as possible.
+ The deployment problems of a protocol dependent on IPsec would be
especially acute in the case of lightweight embedded devices.
Ethernet switches, DSL ``modems,'' and other such devices mostly
do not support IPsec.
+ The API for manipulating IPsec from an application is currently
poorly understood. Writing a program that needs to encrypt some
packets, authenticate some packets, and leave some open -- for the
same destination -- would become more of an exercise in IPsec
rather than in IP measurement.
For the enumerated reasons, we decided to use a simple cryptographic
protocol (based on a block cipher in CBC mode) that is different from
TLS and IPsec.
6.7. Required Properties of MD5
The protocol makes use of the MD5 hash function to convert a
user-supplied passphrase into a key that will be used to encrypt a
short piece of random data (the session key).
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In this document we use cryptographic terminology of [MENEZES].
It has long been suspected, and has been conclusively shown recently
that MD5 is not a collision-resistant hash function. Since collision
resistance was one of design goals of MD5, this casts strong
suspicion on the other design goals of MD5, namely preimage
resistance and 2nd preimage resistance.
OWAMP does not rely on any of these properties.
The properties of MD5 that are necessary are as follows: (1) it is a
function that maps arbitrary length inputs into 128-bit outputs
[fixed-length hash function], (2) a change in any bit of the input
usually results in a change of a few bits of output [weakened
avalanche property], (3) many 128-bit strings have preimages [almost
surjective], and (4) the visible special structure of
natural-language text possibly present in the passphrase is concealed
after application of the function. These are very weak requirements
that many functions satisfy. Something resembling CRC-128 would work
just as well.
We chose MD5 here because it has the required properties and is
widely implemented, understood, and documented. Alternatives would
include (1) a non-cryptographic primitive, such as CRC-128, (2) SHA-1
truncated to 128 bits, or (3) a hash function based on AES (using
Matyas-Meyer-Oseas, Davies-Meyer, or Miyaguchi-Preneel constructions;
we would probably gravitate towards the last one if a block-cipher-
based cryptographically secure hash function were required). Note
that option 1 would not have any cryptographically relevant
properties. We chose not to use it because of lack of
well-documented 128-bit checksums; this specification would incur an
unnecessary burden precisely defining one, providing test vectors,
etc., with no advantage over MD5. Option 2, SHA-1, belongs to the
MD4 family that appears to be under suspicion in light of recent
developments. To avoid creating an impression that any potential
future changes in the status of SHA-1 can affect the status of OWAMP
we chose not to use it. Option 3 would result in a hash function
that, with the current state of knowledge, would probably be one of
the most cryptographically sound. Our requirements 1-4 from the
preceding paragraph, however, do not call for a cryptographically
sound hash function. Just as with CRC-128, this specification would
need to define the hash function and provide test vectors (and
perhaps sample code); we see no advantage in this approach versus
using MD5. (Note that the performance advantages of MD5 are
irrelevant for this application, as the hash is computed on a
relatively short human-supplied string only once per OWAMP-Control
session, so if the Miyaguchi-Preneel construction were documented in
an RFC, we might just as well have used that.)
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INTERNET-DRAFT One-way Active Measurement Protocol December 20046.8. The Use of AES-CBC-MAC
OWAMP relies on AES-CBC-MAC for message authentication. Random IV
choice is important for prevention of a codebook attack on the first
block; it is unimportant for the purposes of CBC-MAC authentication
(it should also be noted that, with its 128-bit block size, AES is
more resistant to codebook attacks than ciphers with shorter blocks;
we use random IV anyway).
IZP, when decrypted, MUST be zero. It is crucial to check for this
before using the message, otherwise existential forgery becomes
possible. The complete message for which IZP is decrypted to non-
zero MUST be discarded (for both short messages consisting of a few
blocks and potentially long messages, such as a response to the
Fetch-Session command).
Since OWAMP messages can have different numbers of blocks, the
existential forgery attack described in example 9.62 of [MENEZES]
becomes a concern. To prevent it (and to simplify implementation),
the length of any message becomes known after decrypting the first
block of it.
A special case is the first (fixed-length) message sent by the
client. There, the token is a concatenation of the 128-bit challenge
(transmitted by the server in the clear) and a 128-bit session key
(generated randomly by the client, encrypted with AES-CBC with IV=0.
Since IV=0, the challenge (a single cipher block) is simply encrypted
with the secret key. Therefore, we rely on resistance of AES to
chosen plaintext attacks (as the challenge could be substituted by an
attacker). It should be noted that the number of blocks of chosen
plaintext an attacker can have encrypted with the secret key is
limited by the number of sessions the client wants to initiate. An
attacker who knows the encryption of a server's challenge can produce
an existential forgery of the session key and thus disrupt the
session; however, any attacker can disrupt a session by corrupting
the protocol messages in an arbitrary fashion, therefore no new
threat is created here; nevertheless, we require that the server
never issues the same challenge twice (if challenges are generated
randomly, a repetition would occur, on average, after 2^64 sessions;
we deem this satisfactory as this is enough even for an implausibly
busy server that participates in 1,000,000 sessions per second to go
without repetitions for more than 500 centuries). With respect to
the second part of the token, an attacker can produce an existential
forgery of the session key by modifying the second half of the
client's token while leaving the first part intact. This forgery,
however, would be immediately discovered by the client when the IZP
on the server's next message (acceptance or rejection of the
connection) does not verify.
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INTERNET-DRAFT One-way Active Measurement Protocol December 20047. IANA Considerations
IANA is requested to allocate a well-known TCP port number for the
OWAMP-Control part of the OWAMP protocol.
8. Internationalization Considerations
The protocol does not carry any information in a natural language.
9. Appendix A: Sample C Code for Exponential Deviates
The values in array Q[] are the exact values that MUST be used by all
implementations (see sections 5.1 and 5.2). This appendix only
serves for illustrative purposes.
/*
** Example usage: generate a stream of exponential (mean 1)
** random quantities (ignoring error checking during initialization).
** If a variate with some mean mu other than 1 is desired, the output
** of this algorithm can be multiplied by mu according to the rules
** of arithmetic we described.
** Assume that a 16-octet 'seed' has been initialized
** (as the shared secret in OWAMP, for example)
** unsigned char seed[16];
** OWPrand_context next;
** (initialize state)
** OWPrand_context_init(&next, seed);
** (generate a sequence of exponential variates)
** while (1) {
** u_int64_t num = OWPexp_rand64(&next);
<do something with num here>
...
** }
*/
#include <stdlib.h>
typedef u_int64_t u_int64_t;
/* (K - 1) is the first k such that Q[k] > 1 - 1/(2^32). */
#define K 12
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break;
V = OWPunif_rand64(next);
for (i = 2; i <= k; i++) {
tmp = OWPunif_rand64(next);
if (tmp < V)
V = tmp;
}
/* Step S4. Return (j+V)*ln2 */
return OWPnum64_mul(OWPnum64_add(J, V), LN2);
}
10. Appendix B: Test Vectors for Exponential Deviates
It is important that the test schedules generated by different
implementations from identical inputs be identical. The non-trivial
part is the generation of pseudo-random exponentially distributed
deviates. To aid implementors in verifying interoperability, several
test vectors are provided. For each of the four given 128-bit values
of SID represented as hexadecimal numbers, 1,000,000 exponentially
distributed 64-bit deviates are generated as described above. As
they are generated, they are all added to each other. The sum of all
1,000,000 deviates is given as a hexadecimal number for each SID. An
implementation MUST produce exactly these hexadecimal numbers. To
aid in the verification of the conversion of these numbers to values
of delay in seconds, approximate values are given (assuming
lambda=1). An implementation SHOULD produce delay values in seconds
that are close to the ones given below.
SID = 0x2872979303ab47eeac028dab3829dab2
SUM[1000000] = 0x000f4479bd317381 (1000569.739036 seconds)
SID = 0x0102030405060708090a0b0c0d0e0f00
SUM[1000000] = 0x000f433686466a62 (1000246.524512 seconds)
SID = 0xdeadbeefdeadbeefdeadbeefdeadbeef
SUM[1000000] = 0x000f416c8884d2d3 (999788.533277 seconds)
SID = 0xfeed0feed1feed2feed3feed4feed5ab
SUM[1000000] = 0x000f3f0b4b416ec8 (999179.293967 seconds)
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
Benjamin Teitelbaum
Internet2
1000 Oakbrook Drive, Suite 300
Ann Arbor, MI 48104
Email: ben@internet2.edu
SIP: ben@internet2.edu
Anatoly Karp
4710 Regent St, Apt 81B
Madison, WI 53705
Telephone: +1-608-347-6255
Email: ankarp@charter.net
Jeff W. Boote
Internet2
1000 Oakbrook Drive, Suite 300
Ann Arbor, MI 48104
Email: boote@internet2.edu
SIP: boote@internet2.edu
Matthew J. Zekauskas
Internet2
1000 Oakbrook Drive, Suite 300
Ann Arbor, MI 48104
Email: matt@internet2.edu
SIP: matt@internet2.edu
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The IETF invites any interested party to bring to its attention any
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rights which may cover technology that may be required to implement
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INTERNET-DRAFT One-way Active Measurement Protocol December 2004
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgments
We would like to thank Guy Almes, Hamid Asgari, Steven Van den
Berghe, Eric Boyd, Robert Cole, Joan Cucchiara, Stephen Donnelly,
Susan Evett, Kaynam Hedayat, Petri Helenius, Kitamura Yasuichi,
Daniel H. T. R. Lawson, Will E. Leland, Bruce A. Mah, Allison Mankin,
Al Morton, Attila Pasztor, Randy Presuhn, Matthew Roughan, Andy
Scherrer, Henk Uijterwaal, and Sam Weiler for their comments,
suggestions, reviews, helpful discussion and proof-reading.
Expiration date: June 2005
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